Conventionally, demands for electric power are progressively increasing, and power systems have become complicated. Therefore, a power stabilization system for stably supplying electric power is necessary, and various apparatuses have been proposed. Also, several apparatuses using a battery module for charging or discharging a power system have been proposed.
Some power stabilization systems described above include a plurality of battery modules, but the ratios of the charge amounts and discharge amounts of these battery modules are the same. Therefore, it may become impossible to properly maintain the stability of a power system, or the remaining life of the charge/discharge count of a given battery module may become much shorter than those of other battery modules.